1. Field of the Invention
This invention relates to cementing of casings in wells. More particularly, method and apparatus are provided for preventing entry of fluids from the surrounding rock into the cement before it cures and for attaining higher radial stress in the cured cement.
2. Background of the Invention
The phenomenon of annular fluid flow (called “annular gas flow” when gas comes to the surface) has long been known to occur during cementing of wells. It is caused by fluids from the surrounding rock entering the wellbore before the cement cures. The resulting loss of control of a well has been responsible for loss of life and property for many years. In addition to the well control issue, annular fluid flow of fluids between zones before the cement cures can cause lack of zonal isolation in wells; water flow to surface from shallow pressurized water sands may occur; and casing shoes may not test at expected pressure integrity. All such occurrences can be manifestations of shortcomings in the primary cementing process.
In 1983, Cooke et al described the results of measurements of pressure and temperature in a curing cement column in seven oil and gas wells (“Field Measurements of Annular Pressure and Temperature During Primary Cementing,” J. Pet. Tech., August 1983). In all the wells, pressure in the cement column began to fall as soon as pumping of the cement ended. The paper explains that the pressure falls because cement shrinks in volume during the curing process because of: (1) the hydration reaction and (2) fluid loss from the cement, and at the same time cement develops a gel strength that prevents the cement column moving downward to compensate for the loss in volume. The decrease in volume combined with the gel formation result in a reduction in pressure in the cement column. If this pressure in cement is reduced to a value below the pressure of a fluid in a permeable rock penetrated by the well before the cement has cured sufficiently, the fluid from the rock enters the cement. This is the phenomenon of “annular fluid flow.” Measurements showed that the pressure in the cement column becomes the same as the pore pressure where fluid has entered. Other laboratory observations showed that fluid entering a cement column may rapidly channel up through the cement. This 1983 paper is hereby incorporated by reference herein for all purposes. Some of the field results reported in the paper were analyzed by Zhou et al (IADC/SPE 59137) using a mathematical model.
U.S. Pat. No. 4,407,365 discloses a method for preventing annular fluid flow—by periodically vibrating the casing while the cement is curing, to maintain pressure in the cement above fluid pressure in the pores of surrounding rock. The patent discloses several methods for vibrating the casing. One method is to ignite small explosives at different depths in the casing. The charges may be run on wire line and set off to cause a plurality of pressure pulses at different depths. The limitation of this method is that the amplitude of any vibrations caused outside the casing would be very small and of very limited extent along the axis of the casing. Another method disclosed is to lock a hydraulic jar attached to a drill string into a retaining groove in the casing and to repeatedly activate and re-set the jar during cement curing time. The limitation to this method is that it would be necessary to run a pipe in the casing after cement is pumped, which would be expensive and time-consuming, and it would be difficult to apply a jarring force in more than one location along the casing. Other methods disclosed include using explosive to propel a projectile against the casing wall, using vibrators on electric wire line, driving vibrators by fluid flow down a pipe string inside casing and electrical or hydraulic hammers. There are at least two disadvantages to the use of vibration sources on a wireline or a pipe string: (1) the wireline or string cannot enter a casing until after cement is pumped, and then delivering the vibration sources to a plurality of preferred depths in the casing would be time-consuming and expensive; (2) the power available for a vibrator would be severely limited by the power transmission capabilities of a wireline. Similar limitations exist for use of explosive charges to propel a projectile against the casing wall. This patent is hereby incorporated by reference herein for all purposes.
Two technical articles that help to elucidate the requirements for a process to maintain pressure in a cement column by vibrating the casing are: (1) “Primary Cementing Improvement by Casing Vibration During Cement Curing Time,” SPE Production Engineering, August 1988, and (2) “The Rheological Properties of Cement Slurries: Effects of Vibration, Hydration Conditions, and Additives” SPE Production Engineering, November 1988. The first article reports that axially vibrating a casing in a 200-ft well with a large electromagnetic vibrator attached to the top of the casing maintained pressure in the cement as it cured and also increased radial stress in the cement, resulting in a very good cement bond log. The increase in radial stress in the cement will increase the resistance to flow between the cement and the wellbore. During the vibration process the surface of the cement in the annulus dropped during each vibration period. The second article reported that breaking the gel structure of cement in a rheometer required only a small amplitude vibration, which was not sensitive to frequency, but that the structure began forming again in a very short time period after it was broken—in the range of 1 minute. Chemical additives in the cement affected gel strength during curing. These two articles are hereby incorporated by reference herein for all purposes.
FIGS. 1A and 1B illustrate why it is critically important in cementing some wells to minimize loss of pressure in the cement after it is pumped and before it cures. FIG. 1A illustrates well 10, which penetrates zones Z1 and Z2. Wellbore 11 has been formed, casing 12 has been placed in the wellbore and cement 13 has been pumped into the annulus outside the casing. The two characteristics of the strata penetrated by the well that are important for cementing are fracture gradient (the pressure gradient that will create a fracture in the earth) and pore pressure. Pressure that can exist in the cement slurry as it is pumped is limited by the fracture gradient in the earth, represented by line 14, on the right. The fracture gradient is represented as slightly less than normal in Zone 1, so this zone will limit pressure in the cement slurry. Cement slurry density and viscosity are selected such that the Equivalent Circulating Density (ECD—line 16) of the cement is less than fracture gradient throughout the cement column and static head is higher than pore pressure in any zone. Pore pressure is represented by line 18, on the left. Pore pressure is slightly higher than normal in Zone 2. In some wellbore conditions, the difference in pressure between highest allowable cement pressure and the highest pore pressure in a zone is small. Therefore, the allowable pressure drop in the cement column before cement pressure drops to pore pressure in a permeable zone may be, for example, only 200-300 psi. Consideration of the fact that cement pressure drops rapidly after pumping in some wells (August 1983 J. Pet. Tech. paper, referenced above) leads to the conclusion that a method to limit pressure drop in the cement after pumping that will keep pore fluids from entering the cement column should be available for application soon after cement-pumping ends. As the cement cures, gel strength increases, which means that breaking gel strength in the cement column, such that the cement will flow, will become more difficult as time-after-pumping increases. Maintaining pressure in the cement column will not only prevent fluid entry into the cement while it is curing, it will also cause flow of cement in a radial direction outward, leading to higher radial stress when the cement has cured.
Later references disclose other methods for vibrating casing during cement curing time. U.S. Pat. No. 5,361,837 discloses a method for preventing annular fluid flow using tube waves in the casing. The tube waves are induced in casing by pressure variations at the surface caused by opening and closing of valves to pump in and out a liquid. The patent discloses that studies showed that casing vibration having a longitudinal displacement of at least 0.25 inches along the wellbore axis is normally more than sufficient to break the gel strength of cement slurry around the region of vibration and that the tube waves can cause longitudinal displacement of about 1.0 to 1.5 inches at the bottom of a casing string. The disclosure posits that extensional waves near the bottom of the casing, in the region of the hydrocarbon zone, are sufficient to prevent annular fluid flow. No evidence is presented, however, that vibration only near the bottom of a casing string will allow the pressure in cement to increase near the bottom of the casing.
U.S. Pat. No. 5,152,342 discloses apparatus and method for vibrating a casing string during cementing, with the vibrating device located near the bottom of the casing string. Cement slurry being pumped down a casing flows through a device, powering the device and causing vibrations in the casing.
U.S. Pat. No. 6,725,923 discloses apparatus that includes hammers that oscillate in a radial direction and hit the wall of tubes when the flexible suspension to which the hammers are attached is pulled. It is stated that the resulting vibrations in the casing can improve cementing.
U.S. Pat. No. 5,377,753 discloses a method for breaking the gel strength of cement in an annulus by applying pressure pulses in a fluid above the annulus.
U.S. Patent Application Publication 2009/0159282 discloses inducing pressure pulses in the cement in the annulus before the cement has cured “for bonding a wellbore to a casing.”
All prior art methods disclosed for inducing vibration into a casing to prevent pressure drop in the cement column have been limited by applying vibration only at the top end or the bottom end of the casing or, if vibration is induced at intermediate points along the casing, by placing apparatus in the casing after pumping of cement has ended (the top plug has been “bumped”). No method or apparatus is known for inducing vibrations into a casing string by sources mechanically coupled to the casing at locations spaced apart along the casing and inducing these vibrations “near simultaneously” (defined herein as within a time period before gel strength of the cement re-builds to its original value after it is broken by vibration), beginning soon after cement pumping ends. What is needed is method and apparatus for inducing an impulse or vibrations at a selected location or at selected locations along a casing string beginning soon after pumping of cement ends and continuing for a selected time during the cement curing period. (“Soon” depends on the time required for the cement to build gel strength to a selected value. For most cements, this time is preferably less than 30 minutes.)